1) Field of the Invention
The invention relates to a riser installation method wherein a riser is lowered from a first vessel, and is towed with its termination end by a tug over the sea bed to a termination point.
2) Description of Related Art
Such an installation method is known from WO 2004/035375 in the name of the applicant. This publication describes a Floating Production Unit (FPU) that is anchored to the seabed and that comprises one or more risers extending from the vessel to the seabed. The FPU comprises lifting means for assembling and lowering risers vertically towards the seabed. The lowered risers can be pulled out from the FPU by a tug boat towards a pre-drilled wellhead and be connected to it so that hydrocarbons can flow from the wellhead to the FPU were the hydrocarbons can be processed and/or stored temporarily.
Another installation method connects different segments of a pipeline on shore and tows the pipeline to the place where it is installed on the seabed. From Offshore Technology Conference OTC 11875, Houston, Tex., 1-4 May 2000 with the title “Hybrid Riser for Deepwater Offshore Africa”, a riser pipe for deep waters is described comprising a steel outer casing with a number of production risers, gas and water injection lines. The riser pipe is assembled on shore and towed to location where it is up righted and connected to the foundation on the seabed. The upper part of the riser is connected to a submerged buoy. After installation of the hybrid riser pipe, the submerged buoy is connected via flexible jumpers to the surface facility such as an FPSO, which may be located at a distance between 70 m-200 m from the submerged buoy. The known method has the disadvantage that during installation of the risers all at once, no hydrocarbon production and/or processing can take place. Furthermore, installation requires special and dedicated installation equipment. Specialised installation vessels are designed to work in as large as possible sea states and are hence, sizeable and costly equipment.
From U.S. Pat. No. 4,182,584 it is known to attach a freestanding marine production riser for use in deepwater between a base portion and a submerged buoy. With a derrick-equipped vessel, such as a semi-sub, the riser casing is lowered through the central part of the buoy and coupled to the bottom until the rigid riser part is completed. Next, a flexible hose is attached to a surface facility for hydrocarbon production and processing. Again, the use of separate, purpose build vessels for riser installation and for hydrocarbon production/processing requires scheduling and mobilising the installation vessel to site at large day rates and the demobilisation of the installation vessel after installation of the riser.
A deepwater field development may consist of several sub sea wells separated by long distances from a centralized FPU. These wells are tied back to the FPU via steel pipelines, and the risers may terminate on the production unit as SCRs. A large field scenario may take years to fully develop. Depending on drilling and completion schedules, construction vessels may be mobilized multiple times in order to connect the wells to the FPU. These vessels can cost millions of dollars to mobilize, and their working rates may exceed one or two hundred thousand dollars per day. It is therefore advantageous to minimize or eliminate the need for these vessels by self-installing the pipelines and risers from the drilling rig vessel.
The method of WO 2004/035375 describes a pipeline installation method without the use of a special pipe lay vessel, the termination part of the pipeline being installed on the sea bed or on a submerged buoy. When attaching the pipeline to a surface vessel, such as a FSU, the departure angle relative to the vertical should be accurately determined to prevent fatigue weakening during use, and should be for instance between 10° and 20°.